Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse source (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled electrical energy sources, the distribution of the electrical energy conveyed to or from the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the electrical energy is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).
For example, with reference to FIG. 1, a neurostimulator may have an output current source 1a (sometimes referred to as an “anodic current source”) and an output current sink 1b (sometimes referred to as a “cathodic current source”) that are configured to supply/receive stimulating current to/from the electrodes Ex, Ey, and ultimately to/from tissue (represented by load 5 having a resistance R). The source 1a and sink 1b are sometimes respectively referred to as PDACs and NDACs, reflecting the fact that the source 1a is typically formed of P-type transistors, while the sink 1b is typically formed of N-type transistors. The use of transistors of these polarities is sensible given that the source 1a is biased to a high voltage (V+), where P-type transistors are most logical, while the sink 1b is biased to a low voltage (V−), where N-type transistors are most logical, as shown in FIG. 1. A suitable current source is disclosed in U.S. Pat. No. 6,181,969 (“the '969 patent”), which is expressly incorporated herein by reference in its entirety.
The output current source 1a and output current sink 1b respectively include current sources 2a, 2b each configured to generate a reference current Iref, and digital-to-analog converter (DAC) circuitry 3a, 3b configured for regulating/amplifying the reference current Iref provided by the current sources 2a, 2b, and delivering output current Iout to the load 5 (having a resistance R). Specifically, the relation between Iout and Iref is determined in accordance with input bits arriving on busses 4a, 4b, which respectively give the output current source 1a and output current sink 1b their digital-to-analog functionality. In accordance with the values of the various M bits on busses 4a, 4b any number of output stages (i.e., transistors M1, M2) are tied together in parallel such that Iout can range from Iref to 2M*Iref.
As shown in FIG. 1 for simplicity, the current source 1a is coupled to an electrode Ex, while the current sink 1b is coupled to a different electrode Ey. However, each electrode may actually be hard-wired to both the current source 1a and the current sink 1b, only one (or neither) of which is activated at a particular time to allow the electrode to selectively be used as either a source or sink (or as neither).
This architecture is shown in FIG. 2, which shows four exemplary electrodes E1, E2, E3, and E4, each having its own dedicated and hard-wired current source 1a and current sink 1b. Thus, the output current source 1a may be associated with electrode E2 (e.g., EX of FIG. 1) at a particular point in time, while the output current sink 1b may be associated with electrode E3 (e.g., EY of FIG. 1) at that time. At a later time, electrodes E2 and E3 could be switched, such that E2 now operates as the sink, while electrode E3 operates as the source, or new sources or sinks could be selected, etc.
Another architecture, shown in FIG. 3, uses a plurality of current sources 1 and sinks 2, and further uses a low impedance switching matrix 6 that intervenes between the sources/sinks and the electrodes EX. Each source/sink pair is hard-wired together at common nodes 7, such that the switching matrix 6 intervenes between the nodes 7 and the electrodes. Of course, only one of the source or the sink in each pair is activated at one time, and thus the node 7 in any pair will source or sink current at any particular time. Through appropriate control of the switching matrix 6, any of the nodes 7 may be connected to any of the electrodes EX at any time. Because all of the available electrodes EX will typically not be activated at one time, the use of the switching matrix 6 decreases the number of current sources needed to supply the electrical current to the activated electrodes EX. Because a relatively large capacitor is typically associated with each current source/sink, decreasing the number of current sources/sinks in any particular architecture is especially advantageous in that it substantially reduces the space needed in the implantable pulse source.
Further details discussing various architectures of current source/sink circuitry are provided in U.S. Patent Publication No. 2007/0100399, which is expressly incorporated herein by reference.
While the use of switching matrices or networks reduces the number of current sources/sinks needed in order to source/sink electrical current to the desired electrodes, a current source/sink is still utilized for each activated electrode. It is, thus, desirable to minimize the number of current sources/sinks needed, while still providing a requisite spectrum of currents to be distributed to the electrodes.